Jet-Lag is the synonym for a number of health-related symptoms that may be caused by flying through several time-zones in long-distance flights. The symptoms are: sleep disturbances, unusual fatigue, decrease in performance and limited wakefulness by day, as well as psychosomatic and gastrointestinal problems. The reason is the temporal discrepancy between the endogenic circadian system (the "interior clock") and external zeitgebers - like the light-dark-cycle, which is - compared to the original local time - altered after a transmeridian flight. These physiological functions are disturbed until the circadian system adapts to the new local time. 

URL for this article